@R-O-N-Y can you share the solution here so that if anyone face this they can refer here

@Kevin_Z yes .i configured according to the guide. Here's the configuration settings screenshot

The QoS doesn't work in my router. i have set the priority (always) of my primary device. My secondary device doesn't have QoS priority enabled. Now whenever i download something on secondary device...

@Kevin_Z ok thank you.. please post a link if possible when it gets updated

@R-O-N-Y strange.. i never faced this issue.. try to order a replacement if it's still in replacement policy.. something is definitely wrong with the router.

@Kevin_Z The link you gave is for EU version..but i have the US version. Here a picture of the back of router for your reference.